This invention relates to circuits for controlling conduction through electromagnetic devices in general and in particular to circuits which permit a limited duration unipolar current to flow through an electromagnetic device coupled to an alternating current potential source.
There has existed for some time the need for portable tools which utilize a single power stroke, such as nailers; staplers; stud drivers; riveters; punches; chisels; and, tackers. In the past, one solution to providing power to such single stroke tools has been the use of pneumatic or hydraulic power. While such a solution has been fairly satisfactory in that large amounts of power can be generated and applied utilizing fluid or gas pressure, their utilization has been limited to those applications in which the amount of use will justify the expense of maintaining and supplying compression facilities.
While an electrically operated tool of the single stroke type would be generally advantageous due to the wide availability of electrical power, the necessity of providing a single power stroke raises certain difficulties in implementation. One previously known approach to providing a single power stroke is the utilization of a rotating electric motor to compress a spring which may be released to drive the stroking portion of the tools. Such springs are subject to fatigue and the mass of electric motor required to stress and release such a spring has made such tools unwieldly and difficult to operate. Another attempt at an electrically powered single stroke tool has utilized a fairly complex electronic circuit and a gate controlled conduction device to limit the polarity and duration of current through an electromagnetic winding; however, these known circuits are less dependable and present possible safety problems within the environment of a hand-held power tool.